The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system with a catalytic converter which comprises a three-way catalyst.
Such a control system is disclosed in U.S. Pat. No. 4,132,199. The system is a feedback control system which comprises an O.sub.2 sensor for detecting the concentration of oxygen in the exhaust gases, an air-fuel mixture supply unit, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit, and an electronic control circuit. The electronic control circuit comprises a comparator for comparing the output signal of the O.sub.2 sensor with a predetermined value, an integrating circuit which is connected to the comparator for integrating the output of the comparator, and a driving circuit connected to the integrating circuit for producing driving pulses for driving the on-off type electromagnetic valve. The O.sub.2 sensor generates an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture induced in the engine cylinder.
The output voltage of the O.sub.2 sensor is higher than a predetermined voltage when the oxygen concentration of the exhaust gases is smaller than a predetermined ratio corresponding to the stoichiometric air-fuel ratio in the air-fuel mixture for the combustion of the mixture and is lower than the predetermined voltage when the oxygen concentration is greater than the predetermined ratio. The duty ratio of the driving pulses varies in dependency on the output of the integrating circuit to correct the air-fuel ratio of the mixture to be supplied to the cylinder to the stoichiometric air-fuel ratio.
On the other hand, in order to detect the engine operation, a sensor for detecting a heavy load of the engine, such as a throttle sensor for detecting the valve opening degree and a vacuum sensor for detecting the vaccum pressure in the induction passage of the engine, is provided and further a sensor for detecting the engine speed or vehicle speed is provided. When the heavy load condition of the engine is detected by such a sensor, the feedback operation is stopped or, the duty ratio is fixed to a predetermined value by the operation of the system, in order to enrich the air-fuel mixture for increasing the output of the engine. However, in accordance with a conventional system, the sensor is actuated in such an operational range where the air-fuel ratio should be controlled. Consequently, the air-fuel ratio is deviated from the stoichiometric value to the rich side which results in improper emission control.
In order to prevent such disadvantages, in an emission control system for an engine having a two-barrel carburetor, the air-fuel ratio control system is provided to control the air-fuel ratio of the mixture only by the primary carburetor in the entire operational range and the secondary carburetor is set to provide a mixture having a rich air-fuel ratio slightly richer than the conventional ratio. The secondary carburetor is designed to operate only in the heavy load (acceleration) condition for providing a rich air-fuel mixture. However, in such a heavy load condition in which the secondary carburetor operates, the air-fuel ratio control system operates to provide the leanest air-fuel mixture in order to dilute the air-fuel mixture, since the secondary carburetor supplies a richer mixture. In the control systems having an on-off type electromagnetic valve for adjusting the amount of air bleed of the primary carburetor, the electromagnetic valve is operated by 100% duty ratio pulses. Therefore, when the engine operation changes from the heavy load condition to a light load condition, the change of the control operation from the 100% duty ratio pulses control to the normal control operation is delayed.
FIG. 4 shows characteristics of an engine in which a dotted line is a border line between the steady state zone and the acceleration zone. When the output of the engine varies as from A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E in FIG. 4, the duty ratio varies as shown in FIG. 5. The duty ratio D decreases gradually to the duty ratio D.sub.1. Therefore, the air-fuel ratio control is delayed, so that the air-fuel mixture is diluted. The hatched range S in FIG. 5 shows the control delay.